Many semi-professional and professional baseball pitchers have noticed differences in elbow motion from one side to the other. The dominant (pitching) arm usually doesn't extend or straighten as much as the nondominant arm.
Sports medicine doctors have suggested this difference occurs because of the player's age or number of innings pitched. They also think maybe arm dominance or history of surgery on the pitching arm makes a difference on elbow motion. But a recent study of 33 pitchers for the Chicago Cubs and St. Louis Cardinals showed no link between elbow range of motion and any of these factors.
We still don't know what causes these changes in elbow motion. But reviewing the records of the pitchers from this same study showed no negative effect of a mild loss of elbow extension on pitchers' stats.